Description.
A very stable and simple FM transmitter circuit is given here. With a matching antenna, this transmitter can attain a range of around 200 meters. I assembled this transmitter few years and got very good results. Let us see how the circuit works.
A condenser microphone (K1) is used to pick the sound to be transmitted. Capacitor C1 is a DC decoupler and the sound signal is coupled to the base of Q1 which wired as a preamplifier.R2 and R3 are the biasing resistors of Q1. Amplified sound signal will be available at the collector of Q1 and it coupled to the emitter of transistor Q2 through the capacitor C4 and 1mH inductor L1. Capacitor C4 decouples the DC component from the preamplifier output. Q2 performs the job of oscillator and modulator. Inductor L2 and variable capacitor C5 forms the tank circuit necessary for creating oscillations. Capacitor C6 is the feedback capacitor. The modulated FM wave will be available at the collector of Q1 and it is transmitted using the antenna A1.Circuit diagram.

Notes.

Assembling the circuit on a good PCB will give better results.

I used a 12V battery for powering the circuit. A 9V PP3 battery will also do the job.

A battery eliminator is also fine but it will induce noise.

Antenna A1 can be a 25cm long copper wire.

K1 is a condenser microphone.

C2 should be a Tantalum capacitor.

Variable capacitor C5 can be used to adjust the transmission frequency.

This simplified formula will help you in making inductors, L = (d²n²) / (18d+40l). Where L is the inductance of the coil in uH , d is the coil diameter in inches, l is the coil length in inches and n is the number of turns. Just try to make the inductor your self using this formaula. If you can’t, then comment here.I will give you the winding details.

Comments

hi jacky, thanks for the circuit, the problem with my case is i want to design a fm transmitter and a reciever in the 50 MHZ band since it is allowed for amateur radio users….can u help me on how to build those both and what r the components that i may require?

can i use a fixed capacitor of 30pf instead of the variable one so that i could get a fixed frequency of 91.9 MHz, which can be atleast heard over a radio. Please reply me as soon as possible. I am in the mid of my project assignment.

Hi suhail, it is an intresting circuit, most of capacitors can be found in scrap radios like 10band world reciever, coil is 3 to 4 turns #22swg or take it from old fm radio. 1mH coil can get from old tv,vcr,dish reciever..etc.

I am facing problems regarding the making of the inductors 0.1uH & 1mH specified in the circuit. Kindly please specify the details, i.e TURNS, LEGTH, DIAMETER & GUAGE OF WIRE. I have bought 24 AWG wire, is it okay??
And also tell me that what range of frequency you had used in your circuit using the tank circuit comprising 0.1uH inductor & 3-10pF capacitor because I have theoretically calculated using the formula ” F = 1/(2*pi*sqrt(L*C))” that the range will be approx 159MHz(for 10pF) to 290MHz(for 3pF) which is beyond the range of commercial FM broadcast band i.e 88MHz to 108MHz.

PLEASE REPLY ME AS SOON AS POSSIBLE TO SORT OUT MY PROBLEMS. ITS VERY URGENT.
THANK YOU & GOOD NIGHT.

L2 should be 3 1/2 turns of 18SWG with an id of 8mm. Coil can be spread or compessed to achieve the broadcast band range. L1 can be standard 1mH choke or honey comb wound 250 turns of silk covered 28 gauge wire on a 3mm former. Since this type of winding will be difficult you can buy a standard one and use it. L2 can be easily wound.

My problem is same with u, I wanted to comfirm about the frequency generated by the cap and inductor, by using a 3~10pf trim cap and the 0.1uH will generate 159mhz~290mhz, totally and clearly above the fm radio receiver broadcast band, should i use 3~10pF trim cap, or use a more capacitance trim cap? pls respond

Hi, just assemble the circuit and use any of 22,33,47 pf fixed cap and any guage wire between 28 to 20 swg is fine and strech or press the coil to get desired frequency, use variable cap to fine tune. A coupling cap is good for ANT.

i m making many projects using this website circuits, All circuit is working nice, so need any help about 200m Fm X-ter, ,transistorized intercom, Em field detector,noise detector ..etc,then send ur req on fb >> solmate solution

Please tell me that where the transmitted signal can be received either fm radio or another receiver circuit for testing and what will be the receiving frequency. Also tell me how we can make the inductor.

sir, I think you are in kerala, as I know you can help me .iam in malappuram district, kerala .idoesnt got the pf and nf capacitors and also the trimmer capacitors what I do now .you know wich shop I got these components

I built this circuit but i could not get any output from the oscillator.
Is the value of c6 really 4.7pf? because when i bought the components, the said that there was no 4.7pf capacitor, so i just replaced it with a 4.7uf capacitor. And is L1 1mh or 1uh? thanks 🙂

Hey guys . I telling in the basis of i have first in eletronics and working in sony as eletronic expert.this site is cheating u. Ucan do this circiut but wOULD NOT WORK.U CAN DOUBTS. Ucan see the rivews .any get the output of the circiut . Every body’s are not working. As u want good working hobby circiuts . U can purchase “HOBBY ELETRONICS” books from shops .its is more better and good. Don’t spoil ur money and time.

Hey guys in the internet so many web site providing hobbycirciut i listened and looked these circiut and it is wrong trust me . I telling in the basis of i have first in eletronics and working in sony as eletronic expert.this site is cheating u. Ucan do this circiut but wOULD NOT WORK.U CAN DOUBTS. Ucan see the rivews .any get the output of the circiut . Every body’s are not working. As u want good working hobby circiuts . U can purchase “HOBBY ELETRONICS” books from shops .its is more better and good. Don’t spoil ur money and time.

Seetharaman sir, I have read all the comments and your respond.before that i bought all the components and made one inductor of 1mh…but…as you replied …to Shine that….L1 should be 1uH (not imH)20 turns of 20SWG (0.9mm dia wire)6mmID covering a length of 20mm…….i tried really hard to built 1 mh’s inductor…..what should i do…i’m using this as my lastyear diploma project..my collage’s project’s sir said to change the project if you are not able to find inductor in a week…and i really want to finish this one….jak.arm1o1@gamil.com
would you plz help me out.

we are connecting the circuit in breadboard and using 0.47 micro henry inductor in place of L2.
what all changes we should make in the circuit for obtaining a freq between 88-108 MHz? And how to check the output?
Plz reply as soon as possible…ITS VERY URGENT…
Thank you

Hi Shashank Solid Tantalum capacitors are close tolerance (+/- 5% or even better)type and of very low internal leakage current, hence most suitable for timer appliocations. As there is no inductance involed most suited for decoupling in high frequency circuits.

Hello Seetharaman,
Can you tellme on thing if i want to use micro controller chip such as HT12 E/D
will it work?
i mean if its not audio signal but digital signal… such as 8 bits with 4
or its only for audio signal ? thanks

Hi Munna the supply voltage 12 volts hence the capacitor w2orking voltage should be greater than that. can 16volt 25volt or even 40volt.
Hi Robert wind L2 with 18SWG super enamelled copper wire with 6mm id and 4 turns, windings spaced at dia of wire. The oscillator center frequency will tune around 100MHz.

Thanks Seetharaman It’s a good idea but I’ve another question. Can you explain me why “Q2 performs the job of oscillator and modulator”? I thought that tank circuit is oscillator but Q1 is amplifire so how transistor can create oscillations?

i would like to ask is air core inductor a compulsory components? can i use other type of inductor such as coloured-band inductor? i assembled this circuit and i didnt get any output from it. can anyone tell me how to troubleshoot the circuit?